Answer:
The best answer to the question: When the ribosome reaches a stop codon on the mRNA, no corresponding tRNA enters the A site. If the translaton reaction were to be experimentally stopped at this point, which of the following would you be able to isolate, would be, A: An assembled ribosome with a polypeptide attached to the tRNA in the P site.
Explanation:
When mRNA is ready to be translated, ribosomal units assemble around the chain to begin the translation. In order for this translation to be correct, and follow a specific, and desired, pattern, the ribosome counts with a series of sites that, when they meet the appropriate signals in the mRNA sequences, it will know when to stop assembling tRNA and with it polypeptides. As such, once a ribosome meets what is known as a stop codon, tRNA and its consequently binding parts of a protein will also stop.
The P site is the second binding site for tRNA in a ribosome. During translation the P site is the one responsible for holding together tRNA and its attaching and growing polypeptide chain. When the process is ended by reaching a stop codon, the bond between the peptydil at the P site and tRNA is broken and the new protein is thus released. Therefore, if in a hypothetical situation, you were to stop the translation process precisely at the point where tRNA has entered the A site, which is the beginning of the translation sequence, you would be able to find the assembled ribosome, and the growing polypeptide chain attached to the already started tRNA on the P site.
